As it's in the tmux tldr, we don't need to mention tmux when talking
about a tmux session. Cleaned up some wording as well.
Oftentimes, users will change their prefix. This shouldn't assume that
your prefix is Ctrl-b.
Detach session also needs to be a lowercase "d" rather than uppercase.
The tip for killing your current session does not work as described.
If you simply use "prefix x", that allows you to kill your pane, not
your session. Killing your session isn't bound to any key command (by
default). So the best way to kill your current session (from within
tmux) is not outlined.
